One Micrometer Length Scale Controls Kinetic Stability of Low-Energy Glasses

Abstract

Alternating current (AC) nanocalorimetry was used to measure the reversing heat capacity C p of low-energy indomethacin glasses as they isothermally transform into the supercooled liquid. As the film thickness increases from 75 to 600 nm, the transformation time increases by more than an order of magnitude, consistent with a surface-initiated transformation mechanism. Eventually, the transformation time becomes constant for films between 1.4 and 30 μm, indicating a distinct bulk transformation pathway. The observation of size-dependent transformation kinetics for glass samples approaching 1 μm is unprecedented. We interpret the crossover in thickness dependence at 1 μm to signify the average distance between transformation initiation sites in the bulk low-energy glass.

In situ investigation of vapor-deposited glasses of toluene and ethylbenzene via alternating current chip-nanocalorimetry

“…Besides varying the temperature varying the film thickness offers another possibility of investigating stable glass transformation kinetics [138]. We varied the film thickness of stable glasses of toluene from 34 nm to 2500 nm and transformed them into the SCL at a certain constant temperature slightly above Tg.…”

“…However, as is observed in Figure 59, log ( ) changes only weakly with temperature and thus even differences inlog(τα) on the order of 1 give negligible differences in log ( ). Nevertheless the differences in the transformation temperature and thus -log(τα) are corrected to fulfill -log(τα) = 0 by assuming the slope of the black line in Figure 59 to be independent of film thicknesses [138]. …”

Origin of Ultrastability in Vapor-Deposited Glasses

“…Additional considerations may be needed to account for the full scope of experimental observations. First, additional studies are needed to connect the surface relaxation time measured in this study with the surface mobility inferred from experiments [2,3,5,[21][22][23]26]. Second, the shape and chemical nature of vapor-deposited molecules can result in preferential orientation within the vapor-deposited film [1,13,[27][28][29][30], while such alignment bias is not expected for ordinary liquid-cooled films.…”

“…Here we use molecular dynamics simulations to model vapor deposition and an efficient Monte Carlo algorithm to determine the deposition rate needed to create ultra-stable glassy films. We obtain a scaling relation that quantitatively captures the efficiency gain of vapor deposition over bulk annealing, and demonstrates that surface relaxation plays the same role in the formation of vapor-deposited glasses as bulk relaxation does in ordinary glass formation.Compared to their liquid-cooled counterparts, vapordeposited glasses often have a higher density [1], a higher kinetic stability [2][3][4], and a lower heat capacity [5]. This makes them promising materials for a wide range of applications, such as drug delivery [6], protective coatings [7,8], and lithography [9].…”

Communication: Surface-facilitated softening of ordinary and vapor-deposited glasses

“…10,11 The kinetic rate law of such a surface-facilitated (SF) transformation is zero-order, i.e., the rate does not depend on the number of transformed constituents. 12 The SF softening of VD glasses is typically observed during alternating-current calorimetry experiments that utilize isothermal holds 13 at temperatures several K above T g (e.g., Fig. 8 of Ref.…”